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Pd Clusters Supported on Amorphous, Low-Porosity Carbon Spheres for Hydrogen Production from Formic Acid Dmitri A. Bulushev,*,†,‡,⊥ Lyubov G. Bulusheva,§,⊥ Sergey Beloshapkin,∥ Thomas O’Connor,†,∥ Alexander V. Okotrub,§,⊥ and Kevin M. Ryan†,∥ †
Chemical & Environmental Sciences Department, University of Limerick, Limerick, Ireland Boreskov Institute of Catalysis, SB RAS, Novosibirsk 630090, Russia § Nikolaev Institute of Inorganic Chemistry, SB RAS, Novosibirsk 630090, Russia ⊥ Novosibirsk State University, Novosibirsk 630090, Russia ∥ Materials & Surface Science Institute, University of Limerick, Limerick, Ireland ‡
S Supporting Information *
ABSTRACT: Amorphous, low-porosity carbon spheres on the order of a few micrometers in size were prepared by carbonization of squalane (C30H62) in supercritical CO2 at 823 K. The spheres were characterized and used as catalysts’ supports for Pd. Near-edge X-ray absorption fine structure studies of the spheres revealed sp2 and sp3 hybridized carbon. To activate carbons for interaction with a metal precursor, often oxidative treatment of a support is needed. We showed that boiling of the obtained spheres in 28 wt % HNO3 did not affect the shape and bulk structure of the spheres, but led to creation of a considerable amount of surface oxygen-containing functional groups and increase of the content of sp2 hybridized carbon on the surface. This carbon was seen by scanning transmission electron microscopy in the form of waving graphene flakes. The H/C atomic ratio in the spheres was relatively high (0.4) and did not change with the HNO3 treatment. Palladium was deposited by impregnation with Pd acetate followed by reduction in H2. This gave uniform Pd clusters with a size of 2−4 nm. The Pd supported on the original C spheres showed 2−3 times higher catalytic activity in vapor phase formic acid decomposition and higher selectivity for H2 formation (98−99%) than those for the catalyst based on the HNO3 treated spheres. Using of such low-porosity spheres as a catalyst support should prevent mass transfer limitations for fast catalytic reactions. KEYWORDS: carbon spheres, formic acid, hydrogen production, functional groups, Pd acetate
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INTRODUCTION The porous structure of catalysts’ supports is important for catalytic applications providing an access of reactants to supported metal nanoparticles and desorption of products. Traditionally, activated carbons containing micropores are used as catalysts’ supports for noble metal clusters. However, the presence of micropores may lead to deteriorated properties of the catalysts complicating molecular transport or even encapsulating metal clusters.1 Another reason for deteriorated properties of carbon supported metallic catalysts can be related to nonuniform particle size distributions. The presence of big metal particles decreases the catalytic surface area and amount of active metal surface sites. The deposition of metal precursors can be enhanced on the external surface of the support particle, leading to the formation of bigger metallic particles as compared to those that are formed on the internal surface of the support. Consequently, a greater understanding of the formation of metal nanoparticles on nonporous and low-porosity carbon supports is of considerable interest. This can be attempted by utilization of carbon spheres with a micrometer or even smaller size. However, the low porosity support should have sufficient concentration of surface anchoring sites for a metal containing © XXXX American Chemical Society
precursor to provide the stabilization of metal clusters and to prevent their sintering. Uniform sized carbon microspheres attract increasing attention of researchers not only as catalysts supports, but also as lubricants, precursors for new materials and composites for electrochemical and energy storage applications.2,3 Serp et al.4 used a chemical vapor deposition of a Pt organic precursor to form metallic clusters over low surface area (0.1− 0.3 μm) carbon spheres catalytically grown from a CH4/H2 mixture at 1373 K. The deposition of Pt on such spheres was possible only after the treatment of the spheres by HNO3. No catalytic tests of this material were performed in this work and neither the concentration of Pt was determined. Recently, Mondal et al.5 studied hydrogenation of ethylene over Pd catalysts supported on core−shell carbon spheres (0.5−1 μm) prepared from acetylene at 1073 K. They discovered a higher activity of Pd on the spheres treated with a H2SO4/HNO3 mixture and related this to observed higher dispersion of Pd on the treated supports. Received: February 3, 2015 Accepted: April 7, 2015
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DOI: 10.1021/acsami.5b00983 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
the oxygen content was determined by difference. The Brunauer− Emmet−Teller (BET) surface area of the spheres was measured by nitrogen adsorption using a Micromeritics Gemini system. Before the measurements, the samples were treated in a flow of nitrogen at 473 K for 2 h. Infrared (IR) spectra were obtained on a PerkinElmer Spectrum 100 unit with an attenuated total reflectance (ATR) accessory. Raman spectra were recorded on a Dilor XY Labram spectrometer equipped with a 20 mW ArHe laser and a Peltier cooled CCD detector. Near-edge X-ray absorption fine structure (NEXAFS) spectra were measured at the Berliner Elektronenspeicherring für Synchrotronstrahlung (BESSY) using radiation from the Russian−German beamline. The C K-edge spectra were acquired in the total-electron yield (TEY) and Auger-electron yield (AEY) modes with a typical probing depth of about 10 nm and less than 1 nm, respectively. The spectra were normalized to the primary photon current from a goldcovered grid recorded simultaneously. The structure of the samples was analyzed using scanning transmission electron microscopy (STEM) on a JEOL JEM-2100F microscope. For STEM imaging the signal from the backscattered electron image (BEI) detector in a secondary electron detection mode was used. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra DLD spectrometer using monochromatic Al Kα radiation of energy 1486.6 eV. C 1s line at 284.8 eV was taken for reference energy calibration. It is known that metallic Pd is covered by a thin oxide layer in ambient air. To remove this, the already reduced Pd catalysts supported on carbon spheres were reduced additionally at 33 Pa of pure hydrogen at 573 K for 30 min in the preparation chamber of the XPS unit and transferred to the measurement chamber in ultrahigh vacuum conditions (10−6 Pa) without contact with air. Catalytic Measurements. For catalytic experiments, 0.0035 g of the Pd catalyst supported on carbon spheres was placed in a quartz fixed bed reactor of 4 mm internal diameter. The sample was reduced in an 1 vol % H2/Ar mixture at 573 K for 1 h and cooled in He to the reaction temperature. An equivalent reductive pretreatment of some catalysts was performed before their transfer in air to different equipment for characterization. Formic acid was introduced into an evaporation volume using a syringe-pump (Sage); the content of formic acid in the created reaction mixture was 2 vol %. Helium was used as a carrier gas. All gases were provided by BOC Gases and were introduced to the system via mass-flow controllers. The reactants and products were analyzed by a gas chromatograph (HP-5890) fitted with a Porapak-Q column and a thermal conductivity detector. The turnover frequency values (TOF) were determined as the ratio of the rate of CO and CO2 formation from formic acid to the number of surface Pd sites, evaluated from TEM determinations of the mean Pd particle size.
The catalytic formic acid decomposition reaction is widely studied, as formic acid can be used for hydrogen storage6 or as a hydrogen donor instead of molecular hydrogen for some hydrogenation reactions.7 Moreover, this acid can be obtained from renewable biomass cellulose by hydrolysis8 or oxidation.9,10 Hence, creation of catalysts very selective to the decomposition of formic acid to hydrogen under mild conditions is important. Several research groups have shown that Pd based catalysts are among the best catalysts for this reaction.11−15 Carbon based materials are mainly used as catalysts supports, as they possess high surface areas, high inertness to the reaction medium and sufficient interaction with noble metal clusters preventing sintering. It is not clear now whether oxygen functional groups created on carbon supports by oxidation in nitric acid or other oxidants can improve or worsen the performance of catalysts in the hydrogen production from formic acid decomposition. High performance of the catalysts can be achieved if the catalyst is in the form of uniform metal clusters of a few nm in size and the reaction is free from mass transport problems. The temperatures normally used to synthesize carbon spheres (higher than 973 K) can lead to graphitization of these materials.2−4,16−18 Wu et al.17 showed that carbon spheres of 0.2−0.5 μm size prepared from acetylene at 1273 K consist of heavily distorted graphene layers. Recently, we have reported that low-porosity uniform amorphous carbon spheres (0.3−1.5 μm) with superhydrophobic properties can be synthesized by pyrolysis of squalane (C30H62) in supercritical CO2 at temperatures as low as 773 K.19 In the present paper, we study further the structure and composition of these original spheres and the spheres treated by HNO3, prior to using them as catalysts’ supports for Pd clusters (2−4 nm) to produce hydrogen effectively from vapor phase formic acid decomposition at low temperatures (
